Absorption is one of the most important gas purification techniques. It involves the transfer of materials from a gas into a liquid. The absorbed material may dissolve physically in the liquid or react chemically with certain compounds of the liquid.
Absorbers used in gas purification include packed columns, plate columns and bubble columns. Plate columns are frequently used. They comprise trays designed for contacting gas and liquid. Such contacting trays may comprise apertures through which both liquid and gas are passed, but also apertures for the passage of gas and downcomers for the passage of liquid. These contacting trays include e.g. bubble cap trays, valve trays and sieve trays. During operation the gas in fed into a lower portion of the column to flow upwards and the liquid is fed into an upper portion of the column to flow downwards. The gas and liquid are contacted with each other during their flows upward and downward, respectively.
Conventional operation of a plate column attempts to avoid back-mixing between trays which may occur as a result of entrainment of liquid droplets or froth by the upwards-flowing gas. It has been described that such entrainment reduces the overall efficiency of the column. In order to avoid entrainment the design of the plate column is usually such that the space between two subsequent contacting trays allows for breaking of the froth formed so that a separation of gas and liquid is obtained and the gas may enter the next contacting tray without entraining substantial amounts of liquid. It is evident that in order to accomplish this separation of gas and liquid, at every tray a substantial amount of essentially void space must be provided in which little mass transfer occurs.
It may be advantageous to create as little of this void space as possible and to accept the backmixing. Especially in cases when an absorbed component of the gas reacts with the contacting liquid, the concentration of absorbed but unreacted material in the liquid is small so that such concentration does not form an impediment to further absorption. Backmixing is acceptable in such cases. It would then be advantageous to have a column which is virtually filled with froth without any substantial void space. In the prior art no recognition is made the backmixing is plate columns is acceptable in such cases.
Bubble columns operate filled with froth substantially without void space. However, they suffer the disadvantage that gas bubble formation occurs only at the gas inlet. On the other hand, in plate columns gas bubbles are reformed at every tray and mass transfer is thus enhanced.
In U.S. Pat. No. 3,992,492 an apparatus for contacting a gas and a liquid is described in which a foam is introduced at the top and gas at the bottom. The foam is entrained by the gas over foam-supporting elements, which are constructed such that a passage is created between the elements which passage is shielded from gas so that a downflow of foam can be obtained. By means of sensors the level of foam in the top of the apparatus is determined. Periodically foam is supplied to the foam-supporting elements. Hence, it appears that this apparatus operates in a batch-type fashion.
It is an object of the present invention to provide an apparatus into which a lean liquid can be fed which can be operated continuously.
It is a further object of the present invention to provide an apparatus which is suitable for gas-liquid contact in which backmixing occurs in a controlled manner.